Synergistic backbone and side chain engineering on crystalline orientation and charge transport of conjugated polymers in field-effect transistors

Abstract

Precise control of molecular packing and crystalline orientation in conjugated polymer thin films is crucial for establishing the structure-property relationship in optoelectronic devices. However, achieving this control remains a significant challenge. Herein, we investigate the synergistic effects of backbone and side chain engineering on molecular packing and crystalline orientation in a series of polythiophene derivatives and establish the correlation between their crystalline structures and organic field-effect transistor (OFET) performance. Specifically, the derivatives include poly (3-dodecylthiophene) (C12), poly (2,5-bis(3-dodecylthiophen-2-yl)thiophene) (T-C12), poly (2,5-bis(3-dodecylthiophen-2-yl)thieno [3,2-b]thiophene) (TT-C12), poly (2,6-bis(3-dodecylthiophen-2-yl)dithieno [3,2-b:2′,3′-d]thiophene) (TTT-C12) with different unsubstituted thiophene units in the backbone, along with TTT derivatives bearing varying alkyl side chains (hexyl, decyl, and tetradecyl, denoted as TTT-C6, TTT-C10, and TTT-C14, respectively). Remarkably, TT-C12, with its moderate thiophene units, facilitates the formation of edge-on crystallites with enhanced ordering and orientation. Moreover, TTT-C14, with the longest alkyl side chains, promotes more ordered edge-on crystallites than the other two TTT-C6 and TTT-C10. The charge transport characteristics of these polymers are closely linked to variations in their crystalline structures. This work underscores the effectiveness of molecular engineering in tailoring the crystalline orientation and charge transport. As a model system, the deep insights into the structure-property relationships of polythiophene derivatives can be extended to other conjugated polymers for high-performance optoelectronic devices.